This invention relates generally to the production of single crystal silicon ingots, and in particular to a method for stacking and melting chunks of polycrystalline silicon source material in a crucible as part of the Czochralski method to produce crystal ingots of improved quality.
Most semiconductor chips used in electronic devices are fabricated from single crystal silicon prepared by the Czochralski process. In that process, a single crystal silicon ingot is produced by melting polycrystalline silicon source material stacked within a quartz crucible, stabilizing the crucible and source melt at an equilibrium temperature, dipping a seed crystal into the source melt, withdrawing the seed crystal as the source melt crystallizes on the seed to form a single crystal ingot, and pulling the ingot as it grows. Melting occurs at temperatures higher than 1420.degree. C., in an inert gas environment at low pressure. The crucible is continually rotated about a generally vertical axis as the crystal grows. The rate at which the ingot is pulled from the source melt is selected to form an ingot having a desired diameter.
A substantial concern in the production of single crystal ingots by the Czochralski process is the need to prevent formation of dislocations, voids, or other defects in a crystal lattice structure. Dislocations can be generated by one or more of a variety of possible causes, including thermal shocks, vibrations or mechanical shocks, internal strain due to regional cooling rate differences, solid particles in the melt at the crystal growth interface, gas bubbles trapped within the melt, and surface tension effects. Once generated, dislocations degrade the uniformity of electrical characteristics and permit the attachment of impurities. Voids or air pockets within the crystal similarly disrupt continuity of the material.
Further aggravating the problem is that any localized defect or dislocation in the crystal spreads widely and usually renders the product unusable. If a first dislocation forms in the crystal structure, it multiplies and generates numerous dislocations which spread out into the crystal. Therefore, if a dislocation-free growing crystal is disturbed at one point, the whole cross section and a considerable part of the already grown crystal may be inundated with dislocations. Thus it is crucial to maintain the growing crystal in a completely dislocation-free state. Otherwise, production yield and throughput will be substantially degraded.
The initial stacking, or charging, of polycrystalline silicon source material into a crucible and the melting thereof must be done carefully, or the crystal will be ruined. The typical ways that defects are inadvertently introduced during charging and melt-down are through damage to the crucible, splashing of molten silicon, and trapping of gas pockets.
Damage to the crucible occurs when solid silicon strikes or scratches a wall. Crucibles have walls lined with quartz that minimize reaction with the molten silicon bath and thereby avoid the introduction of impurities. Polycrystalline silicon source material is usually supplied in irregularly shaped chunks of varying size having edges that may be smooth, blunt, or sharp. When the crucible is initially charged with chunk polycrystalline silicon, sharp or pointed edges can gouge the quartz walls. When the crucible is fully charged, the weight of the load rests on the chunks along the bottom walls, which can scratch and nick the walls, especially when the chunks are moved or shift under load. Particles of quartz typically flake off or break off from damaged crucible walls and fall into the silicon source material. These particles become fused silica impurities that float on the silicon melt or attach to the liquid/solid crystal growth interface, significantly increasing the likelihood of a dislocation forming within the crystal.
As melting proceeds, splashing may occur. Chunks along the bottom melt first, and subsequently chunks above shift or fall into the melt. If the heat distribution in the crucible is not uniform, the lower portion of the chunk-polycrystalline silicon can melt away and leave either a "hanger" of unmelted material stuck to the crucible wall above the melt or a "bridge" of unmelted material bridging between opposing sides of the crucible wall over the melt. When the charge shifts or a hanger or bridge collapses, it may splatter molten silicon on the crucible walls above the surface of the melt. Thermal shock to splattered regions damages the crucible, leading to subsequent contamination of the source melt with quartz particles and dislocations in the crystal ingot.
The way the crucible is charged with polycrystalline silicon chunks may also lead to trapping gas pockets in the melt, another source of crystal defects. As melting progresses molten silicon flows from the edges toward the bottom center of the crucible, gradually filling the crucible with liquid. Ideally, the gas between chunks in the crucible, which is typically an inert gas such as argon, is merely displaced by the liquid and stays above the liquid surface of the melt. However, when a relatively large or flat chunk of solid silicon is located adjacent the bottom of the crucible, oriented generally parallel the bottom so that a flat side of the chunk does not conform to the curved bottom surface of the crucible but rather is spaced from the surface, pockets of gas can be trapped beneath the chunk as melting proceeds. Chunks or particles of solid silicon that are very small can aggravate the problem by creating void space beneath or between other chunks in which gas bubbles become trapped. Further, small chunks are slow to melt because they have greater relative surface area. Although sometimes gas pockets rise to the top surface of the source melt and are expelled at some point during the melt-down, often they stay in the melt, later becoming voids or air pockets within the single crystal ingot.
Another factor contributing to the creation of trapped gas pockets is the equal relative ambient pressures at which melting and stabilization proceed. Typically in the Czochralski process, the steps of melting the silicon and stabilizing to a thermal equilibrium temperature occur at close to the same ambient pressures; typical levels being 23 Torr absolute (standard sea level atmospheric pressure is 760 Torr). However, this equal-pressure practice has a drawback in that any bubbles of argon gas that become trapped within the source melt during melt-down have no relative pressure impetus during stabilization to expand or move to the surface of the melt for release. Bubbles in the source melt, both those along the bottom walls and those that are suspended within the melt, remain at a pressure equal to the ambient gas above the source melt, and are urged to the surface only by natural buoyancy. Because the source melt is viscous, buoyancy may be insufficient to draw many bubbles to the surface of the melt for expulsion. Gas bubbles remain in the melt, and these bubbles often attach at the crystal growth interface where they cause the formation of voids.